Again in August of 1938 they had to resort to time honored practice of surcharging old high-valued stamps to the 5-30k values the tuvan post needed for postal duty.  The surcharges done at this time obliterated the old denominations. There were six values overprinted in black. 

Here we enter the most difficult phase of Tuvan philately...these surcharged issues existed almost entirely behind the "iron curtain" until recently, and in any case only a handful of examples of each exist.